The apparatus of the present invention generally relates to power supplies and more particularly to power converters for providing direct current (DC) power.
In present day electronic equipment, the primary energy source is usually that received from a public utility. The voltage provided must be converted to DC at different output voltage levels. The converter must operate with wide variations in load current drawn. It must work into an open circuit. It must also withstand a short circuit at its output terminals with no destruction of its parts or other malfunction. Recovery to normal operation must be automatic upon removal of the fault.
Of all the semiconductor switch elements available today, the silicon controlled rectifier (SCR) has higher voltage and current capabilities. It requires a relatively simple and power efficient gate drive circuit to put it in conduction.
However, detracting from the superior attributes of the SCR is the fact that once turned on, it cannot be turned off by gate action. It must be turned off by causing its anode current to fall below the holding level. This must be done so the SCR can recover its forward blocking ability. Hence, lack of gate turn off control is a major problem in any converter design utilizing SCRs. This has resulted in the paralleling of bipolar transistors for power conversion applications at the kilowatt level.
Turn-off control can be achieved by using resonant circuits in which natural circuit oscillations commutate or turn off the SCR in each cycle. The two basic types of resonant converters are the parallel resonant circuit and the series resonant circuit. The SCR parallel resonant converter has the load in parallel with the capacitor (C) or inductor (L) of the LC resonant circuit. It has decent open circuit operation characteristics, but a short circuit at the output terminals is reflected back to the reactive element across which it is connected and prevents continued oscillation. Even an overload which is less than a dead short circuit can suddenly extrac t so much energy from the energy circulating in the resonant circuit that there is not enough left to complete the cycle of oscillation and commutate the SCR before additional energy can flow in from the source. The SCR latches on and oscillation stops.
The SCR series resonant converter has the load in series with the LC resonant circuit. It has excellent short circuit characteristics but when the load is open circuited, the output voltage either goes very high or the circuit stops oscillating completely, depending on the magnitude of the circuit losses which act as a partial load. Not only does the output voltage go high with a reduction in load drawn from the converter, but the resonant frequency changes drastically. For the series circuit, resonant frequency (F) is: ##EQU1## where R is load resistance, L is inductance, and C is capacitance. Dewen and Shively in a paper entitled "An Output Clamped Series Inverter" delivered at the IEEE Power Equipment Specialists Conference in 1979, show calculations proving that with only a modest two to one change in the value of load resistance, resonant frequency also changes by approximately a two to one factor. Any abrupt load change that lowers the resonant frequency and thus upsets the critical timing relationship between the oscillation and the SCR gate pulse train causes the SCR to latch on, thus preventing continued oscillation.
The shortcomings of the SCR resonant converter have been the subject of circuit developments by various researchers which are described hereafter as best understood.
In U.S. Pat. No. 4,024,453 (Corry), the short circuit problem in a parallel resonant converter is solved by introducing an added inductor in the resonant circuit to raise the frequency of operation so that the circuit will continue to commutate. Obviously, normal frequency of operation is below the capability of the SCR. This is a shortcoming since the SCR must operate at the highest frequency it is capable of in order to compete with transistors in small size applications.
In U.S. Pat. No. 4,042,871 (Grubbs et al), the problem of impending commutation failure due to an overcurrent situation condition is overcome through the use of added circuitry to monitor input current and take preventive action. The current sensing resistors cause a reduction in overall power conversion efficiency. Here also, a race against time must be won.
In U.S. Pat. No. 4,069,449 (Farnsworth), complex circuitry is added to solve the overload problem. Again, speed is of the essence.
In U.S. Pat. No. 4,156,274 (Fukui et al), the open circuit problem in a series resonant converter is overcome by use of an added transformer winding and an SCR to clamp the output voltage to the DC input line. Again, increased parts count and circuit complexity result.
In U.S. Pat. No. 4,200,830 (Oughton et al), the overcurrent problem in a series resonant converter is solved by means of a fault current sensor and feedback arrangement. The result is a relatively complex circuit and increased parts count.
Referring again to the previously referenced article by Dewan and Shively, a series resonant converter is described. The open circuit problem is dealt with by means of an added tertiary winding on the output transformer which is clamped to the DC input voltage bus by means of a rectifier bridge. If the DC input is unregulated, the clamp level will fluctuate reducing the effectiveness of this method. Again, this solution suffers from added circuit complexity and added components.
In General Electric Co. Application Note 200.49 Feb. 1961, entitled "A Low Cost Ultrasonic Frequency Inverter Using A Single SCR", N. Mapham, a parallel resonant inverter is described. A separate inductor and capacitor are added to the circuit solely to limit current to a safe value during an overload. The capacitor has 66 times the capacitance used in the resonant circuit and the inductor has 100 times the inductance of the inductor used in the resonant circuit. The addition of these two large reactive elements is not a good solution to the short circuit problem if size, weight, and cost are design requirements.
It is accordingly a primary object of the present invention to provide an improved power converter for generating a DC output.